There are many forms of the silicon material generally used in semiconductors, such as amorphous, polycrystalline and single crystalline silicon. Polycrystalline silicon (polysilicon) thin film has lately attracted considerable attention due to its special physical properties and low cost in thin film transistor (TFT) fabrication, especially in the application of thin film transistor liquid crystal displays (TFT-LCD).
The electrical performance of polysilicon is better than that of amorphous silicon but worse than that of single crystalline silicon. Polysilicon is an aggregate of single crystal grains and thus there are many grain boundaries; so grain size enlargement and grain boundary reduction for polysilicon are very important for improving device performance.
For the field of display technology, it is highly focused to develop a flat panel display with higher performance (e.g., System On Panel (SOP)). And therefore, it is necessary to improve the electrical performance of polysilicon thin film transistors. For example, higher carrier mobility of thin film transistors is helpful for higher resolution, higher response speed, higher open ratio, and lower power consumption.
The conventional method for fabricating polysilicon film is solid phase crystallization (SPC), but SPC is not applicable to flat panel display fabrication because the upper-limit process temperature of a glass substrate is 650° C. Besides, the direct chemical vapor phase deposition (CVD) method is also used. In SPC and CVD, the grain size of polysilicon is as small as 100 nm, and therefore the performance of polysilicon film is limited.
The excimer laser annealing (ELA) method is currently the most commonly used polysilicon film fabrication method. In ELA, the grain size of polysilicon is about 300-600 nm, and therefore the carrier mobility of polysilicon film reaches about 200 cm2/V-s. However, ELA is still not sufficient for future flat panel displays with high performance. Besides, the grain size distribution is not uniform because of irregular laser energy deviation, and the electrical performance of devices, such as carrier mobility and uniformity of threshold voltage, is decreased.
The performance of devices depends on the quality of the polysilicon film; crystal grain size affects the carrier mobility directly. The existence of grain boundaries leads to a rise in threshold voltage and leakage current, and a decrease in carrier mobility and device stability. So in addition to trying to enlarge the crystal grain size, uniformity of grain size distribution and grain location order control are also ways of decreasing the grain boundary effect in channels for improving device performance.